1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly, to a liquid crystal display (LCD) device and a method of fabricating an LCD device.
2. Discussion of the Related Art
As demand for various display devices increases, development of various type of flat display devices, such as LCD device, plasma display panel (PDP) device, electroluminescent display (ELD) device, and vacuum fluorescent display (VFD) device, has increased. Among these various flat display devices, LCD devices have been commonly used because of their thin profile, light weight, and low power consumption. For example, LCD devices are commonly used as a substitute for cathode ray tube (CRT) devices. In addition, LCD devices are commonly used in notebook computers, computer monitors, and televisions. However, in order to use LCD devices in general display devices, the LCD devices must be produce high quality images, such as high resolution and high luminance with a large-sized screen, while still maintaining their light weight, thin profile, and low power consumption.
FIG. 1 is a schematic perspective view of an LCD device according to the related art. In FIG. 1, an LCD device includes first and second substrates 1 and 2, and a liquid crystal layer between the first and second substrates 1 and 2 formed by an injection method. The first substrate 1 includes a plurality of gate lines 4 arranged along a first direction at fixed intervals, a plurality of data lines 5 arranged along a second direction perpendicular to the first direction at fixed intervals, a plurality of pixel electrodes 6 arranged in a matrix configuration within pixel regions P defined by crossing of the gate and data lines 4 and 5, and a plurality of thin film transistors T enabled according to signals supplied to the gate lines 4 for transmitting signals from the data lines 5 to the pixel electrodes 6. The second substrate 2 includes a black matrix layer 7 that prevents light from portions of the first substrate 1, except for the pixel regions P, an R/G/B color filter layer 8 for displaying colored light, and a common electrode 9 for producing images.
In FIG. 1, since the liquid crystal layer 3 is formed between the first and second substrates 1 and 2, liquid crystal molecules of the liquid crystal layer 3 are driven by an electric field generated between the pixel electrode 6 and the common electrode 9. For example, an alignment direction of the liquid crystal molecules of the liquid crystal layer 3 is controlled by the induced electric field thereto. Accordingly, light irradiated through the liquid crystal layer 3 may be controlled by the alignment direction of the liquid crystal molecules, thereby displaying images. The LCD device of FIG. 1 is commonly referred to as a twisted neumatic (TN) mode LCD device, which has disadvantageous characteristics, such as narrow viewing angles.
In order to overcome these problems of the TN mode LCD device, an in-plane switching (IPS) mode LCD device has been developed. In the FPS mode LCD device, a pixel electrode and a common electrode are formed in a pixel region in parallel to each other at a fixed interval therebetween. Accordingly, an electric field parallel to substrates is generated between the pixel electrode and the common electrode, thereby aligning liquid crystal molecules of a liquid crystal layer by the electric field parallel to the substrates.
FIGS. 2 and 3 are flow charts of method for fabricating an LCD device according to the related art, wherein FIG. 2 shows a liquid crystal injection method and FIG. 3 shows a liquid crystal dispersion method.
In FIG. 2, the method for fabricating an LCD device is divided into three processes, including an array process, a cell process, and a module process. The array process largely includes two steps of forming a TFT array having gate and data lines, a pixel electrode, and a thin film transistor on a first substrate, and forming a color filter array having a black matrix layer, a color filter layer, and a common electrode on a second substrate. During the array process, a plurality of LCD panels are formed on one large-sized glass substrate, and the TFT array and the color filter array are formed within each of the LCD panels. Then, the TFT substrate and the color filter substrate are moved to a cell process line. Subsequently, an alignment material is coated on the TFT substrate and the color filter substrate, and an alignment process (i.e., rubbing process) S10 is performed to the substrates to obtain a uniform alignment direction of liquid crystal molecules. At this time, the alignment process S10 is carried out in order of processes for cleaning before coating of an alignment layer, printing the alignment layer, baking the alignment layer, inspecting the alignment layer, and rubbing the alignment layer. Accordingly, the TFT substrate and the color filter substrate are respectively cleaned (S20).
Then, ball spacers for maintaining a cell gap between the two substrates are scattered on one of the two substrates (S30), and a seal pattern is formed corresponding to the circumference of respective LCD panel regions to bond the two substrates to each other (S40). At this time, the seal pattern includes a liquid crystal injection inlet through which liquid crystal material is injected. The ball spacers are formed of plastic balls or minute elastic particles. Then, the TFT substrate and the color filter substrate having the seal pattern therebetween are position to oppose each other and bonded to each other, and then the seal pattern is hardened (S50).
Then, the bonded TFT and color filter substrates are cut into individual LCD panel regions (S60), thereby manufacturing the unit LCD panels each having a fixed size. Subsequently, the liquid crystal material is injected to the LCD panel through the liquid crystal injection inlet, and the liquid crystal injection inlet is sealed (S70), thereby forming a liquid crystal layer.
After an inspection process (S80) for observing external appearances and testing for electric failures in the LCD panel is performed, the process of manufacturing the LCD device is completed.
During the process for injecting the liquid crystal material, the LCD panel and a container having liquid crystal material therein are provided within a vacuum chamber. Accordingly, moisture and air bubbles in the liquid crystal material and the container are simultaneously removed, and an interior space of the LCD panel is maintained in a vacuum state. Then, the liquid crystal injection inlet of the LCD panel is dipped into the container having the liquid crystal material in the vacuum state, and the vacuum state inside the chamber is changed to an atmospheric pressure. Thus, the liquid crystal material is injected into the interior of the LCD panel through the liquid crystal injection inlet according to a pressure difference between the interior of the LCD panel and the vacuum chamber.
However, the injection method has the following disadvantages. First, after cutting the large-sized glass substrate into the LCD panel regions, the liquid crystal injection inlet is dipped into the container having the liquid crystal material while maintaining the vacuum state between the two substrates. Thus, significant amounts of time are required for injecting the liquid crystal material between the two substrates, thereby lowering production yield. When forming large-sized LCD devices, it is difficult to completely inject the liquid crystal material into the inside of the LCD panel, thereby causing the failure due to incomplete injection of the liquid crystal material. Furthermore, significant amounts of time are required for injecting the liquid crystal material into large-sized spaces for large-sized LCD devices.
In order to overcome these problems of the liquid crystal injection method, the liquid crystal dispersion method has been developed, in which two substrates are bonded to each other after dispersing liquid crystal material on any one of the two substrates. In FIG. 3, before bonding the two substrates, the liquid crystal material is dispersed on any one of the two substrates. Accordingly, it is impossible to use ball spacers for maintaining a cell gap between the two substrates since the ball spacers move along a dispersion direction of the liquid crystal material. Thus, instead of the ball spacers, patterned spacers or column spacers are fixed to the substrate to maintain the cell gap between the two substrates. As shown in FIG. 3, during an array process, a black matrix layer, a color filter layer, and an overcoat layer are formed on the color filter substrate. Then, a photosensitive resin is formed on the overcoat layer, and selectively removed to form the column spacer on the overcoat layer above the black matrix layer. The column spacers may be formed in a photo process or an ink-jet process.
Then, alignment layers are respectively coated on entire surfaces of the TFT substrate and the color filter substrate including the column spacers, and a rubbing process is performed thereto. After cleaning the TFT substrate and the color filter substrate (S101), the liquid crystal material is dispersed on one of the two substrates (S102), and a seal pattern is formed in the circumference of an LCD panel region on the other of the two substrates by a dispensing device (S103). At this time, it is possible to perform dispersion of the liquid crystal and formation of the seal pattern on any one of the two substrates.
After the other substrate having no dispersion of the liquid crystal material is inverted (S104), the TFT substrate and the color filter substrate are bonded to each other by pressure, and the seal patterned is hardened (S105). Subsequently, the bonded substrates are cut into the respective LCD panels (S106). In addition, an inspection process (S107) for observing external appearances and tests for electric failures in the LCD panel are performed, so that the process of manufacturing the LCD device is completed.
In the method of manufacturing the LCD device according to the liquid crystal dispersion method, the column spacer is formed on the color filter substrate, and the liquid crystal material is dispersed on the TFT substrate, then the two substrates are bonded to each other, thereby forming the LCD panel. Accordingly, the column spacer is fixed on the predetermined portion of the color filter substrate. In addition, the column spacer having a predetermined height is in contact with the predetermined portion of the TFT substrate corresponding to the gate or data line.
However, the column spacer of the LCD device according to the liquid crystal dispersion method causes the following problems to the LCD panel. For example, when the LCD device is formed by the liquid crystal dispersion method, the column spacers are formed on the color filter substrate corresponding to the gate or data line. Accordingly, the column spacers are formed at the same height to be corresponding to the line region having the same width (gate or data line). In addition, the columns spacers having the same height are formed on the color filter substrate in opposite to the TFT substrate, and the two substrates are bonded to each other. Since the supportive force of the column spacers is weak, the LCD panel may suffer from problems due to gravity. For example, when the LCD device is at a high temperature, the LCD panel may have a protruding portion because the liquid crystal material has large thermal expansion characteristics. When the LCD panel is placed in a vertical direction, the liquid crystal molecules of the LCD panel migrate to the lower-corner direction, thereby causing a gathering of liquid crystal molecules to the predetermined portion on the LCD panel due to the effects of gravity.
FIG. 4A is a cross sectional view of a color filter substrate having column spacers according to the related art, and FIG. 4B is a cross sectional view of bonded TFT and color filter substrates according to the related art. In FIG. 4A, a plurality of column spacers 20 are formed on a black matrix layer (not shown) of a color filter substrate 2 at fixed intervals, wherein each of the column spacers 20 is formed at a height “h.” Then, as shown in FIG. 4B, the color filter substrate 2 having the column spacers 20 thereon is bonded to a TFT substrate 1. Accordingly, the height “h” of the column spacer 20 decreases to a height “h′” due to pressure created during the bonding process.
As shown in FIGS. 4A and 4B, after the bonding process, the column spacer 20 of an LCD panel 10 has the height “h′” corresponding to a cell gap. Accordingly, when the liquid crystal material expands at high temperatures, the column spacer 200 compensates the supportive force for the TFT substrate 1 and the color filter substrate 2 in the extent corresponding to a thickness difference “h-h′” between the height “h” of the column spacer 200 and the cell gap “h′.” Thus, the thickness difference “h-h′” between the height “h” of the column spacer 200 and the cell gap “h′” corresponds to a margin to compensate for gravity.
In FIGS. 4A and 4B, the column spacers 20 are formed on the portions corresponding to the line regions having the same width, whereby the thickness difference “h-h′” is limited to 0.1 mm to 0.15 mm. Furthermore, since the column spacers 20 are patterned, the column spacers 20 may each have slightly different heights, whereby it is impossible to obtain uniformity of gravity on the entire regions of the LCD panel. As compared with ball spacers each having a spherical end, the column spacer has a larger contact area with the substrate, thereby generating significant frictional forces between the column spacer 20 and the substrate. Accordingly, if a screen of the LCD device having the column spacers 20 is touched, spots will be generated on the screen and will remain for a long time.
FIG. 5A is a plan view of an LCD device according to the related art, and FIG. 5B is a cross sectional view along I-I′ of FIG. 5A according to the related art. In FIG. 5A, if an LCD panel 10 is continuously touched with a finger along a predetermined direction, the upper substrate 2 of the LCD panel is shifted at a predetermined interval along the touch direction, as shown in FIG. 5B. When the cylindrical column spacers are in contact with the lower and upper substrates 1 and 2, they cause significant frictional forces between the column spacers and the two opposing substrates. Thus, the liquid crystal molecules between the column spacers are not restored to their original states, thereby generating spots on the screen. In addition, when the LCD panel is touched with the finger along the predetermined direction, as shown in FIG. 5B, the liquid crystal molecules gather within the region around the touched portion, whereby the region around the touched portion protrudes. In this case, the cell gap “h1” corresponding to the protruding portion is higher than the cell gap “h2” of the remaining portions, thereby causing light leakage. Meanwhile, since the touched portion has no liquid crystal molecules, blurred portions appear on the screen in a black state, thereby deteriorating luminance of the LCD panel 10. Furthermore, the ball spacers are formed on the substrate in a large amount, but the column spacers are selectively formed on the predetermined regions of the LCD panel. Accordingly, when the LCD panel is pressed at a predetermined portion having no column spacers, the substrates bend and form a hollow state due to low restoring speed of the substrates, thereby generating spots on the screen of the LCD panel.